This invention relates to electrochemical sensing circuits and in particular to electrical circuits which sense cell current flow from a three terminal electrochemical cell.
Three terminal electrochemical cells are used for a variety of gas monitors and comprise a cell in which a gas to be analysed is introduced and three spaced apart electrodes. The three electrodes comprise a main pair across which the cell current is generated and a reference electrode which enables a potential at a predetermined point in the cell electrolyte measured. The cell current is proportional to the concentration of the compound or element being sensed by the cell, which may, for example, be carbon monoxide.
Known three-terminal electrochemical cells can be stabilised using the circuit shown in FIG. 1. In order to stabilise the cell, the "working" and "reference" electrodes, labelled "W" and "R" respectively must be brought to the same electrical potential. No current is taken from the reference electrode. Instead, current is injected into the counter electrode, labelled "C", by the amplifier Al until both the reference and working electrodes, R and W respectively, are the same potential. The current which flows in both the counter and working electrodes is the cell current and due to the internal operation of the cell this is proportional to the concentration of the compound being sensed by the cell.
Referring to FIG. 1, the Amplifier, A1, maintains the reference electrode at 0V by feeding back current to the counter electrode. Amplifier A2 maintains the working electrode at 0V, since the negative input of amplifier A2 is at 0V. The cell current is driven by amplifier A1 but is sensed by amplifier A2, because the cell current passes through resistor R2 to develop the voltage V out.
A disadvantage of the prior known circuit shown in FIG. 1 is that it is prone to oscillation, because the virtual earth impedance of each amplifier appears as part of a feedback path of the other amplifier. This can lead to oscillation at high frequencies, where the virtual earth impedances are not well defined.
A second disadvantage, for low cost microcontroller-based applications, is that the output V out is an analogue voltage which must go through an analogue to digital conversion before it can be processed digitally.
A third disadvantage is that, whereas V out is normally positive when gas is being sensed, the counter electrode charges negatively, requiring the output of amplifier A1 to go negative. Therefore the circuit shown in FIG. 1 requires both positive and negative supplies (shown as V+ and V-).
A further cell which uses a potentiostat-type circuit is described in U.S. Pat. No. 4,048,041 (U.S. Army). The electronic circuit controls voltage potential applied to working electrodes of a three-electrode electrochemical cell. The electrochemical cell is incorporated into a sensor which operates by measuring the difference current between the cell's grounded anode and a negatively pulsed cathode. This circuit is quite complex, and requires both positive and negative supplies.